1. Field of the Invention
The present invention relates to a semiconductor laser device having a compressive strain active layer.
2. Description of the Related Art
The active layers in the conventional semiconductor laser devices which are formed on a GaAs substrate and emit laser light having a wavelength of 0.9 to 1.2 micrometers have a composition which causes a compressive strain. Since the compressive strain produces crystal defects or the like, the above semiconductor laser devices cannot attain to satisfactory characteristics and high reliability in operation with high output power.
U.S. Pat. No. 5,671,242 (corresponding to Japanese Unexamined Patent Publication, No. 8(1996)-78786) discloses a stress-compensation semiconductor laser device including a stress-compensation strained quantum well layer in which compressive strain well layers and tensile strain barrier layers are alternately laminated so that the average strain of the entire active layer is a compressive strain. However, when the amount of the compressive strain is increased in this semiconductor laser device, the difference in the strain between the compressive strain well layers and the tensile strain barrier layers increases, and therefore the strong interlayer stress is produced. Thus, it is impossible to achieve satisfactory crystallinity without producing defects in vicinities of the boundaries between the layers.
T. Fukunaga et al. (xe2x80x9cReliable operation of strain-compensated 1.06 xcexcm InGaAs/InGaAsP/GaAs single quantum well lasers,xe2x80x9d Applied Physics Letters, Vol. 69, Issue 2, pp. 248-250, 1996) report a semiconductor laser device in which an InGaAs strained quantum well active layer is formed above a GaAs substrate, and tensile strain barrier layers are formed adjacent to the InGaAs strained quantum well active layer so as to compensate for the strain in the InGaAs strained quantum well active layer. Although the reliability of the semiconductor laser device is improved by the provision of the tensile strain barrier layers, the above semiconductor laser devices cannot attain to a practical reliability level or high output characteristics.
An object of the present invention is to provide a semiconductor laser device which can compensate for compressive strain in an active layer, and is reliable in a wide output power range from low to high output power.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device comprising a substrate and an active region being formed above the substrate. The active region includes an active layer and optical waveguide layers. The active layer has a predetermined amount of compressive strain and a predetermined thickness, and the optical waveguide layers have a predetermined amount of tensile strain and a predetermined total thickness, and are formed so that the active layer is sandwiched between the optical waveguide layers. The absolute value of the sum of the product of the predetermined amount of compressive strain and the predetermined thickness of the active layer and the product of the predetermined amount of tensile strain and the predetermined total thickness of the optical waveguide layers is equal to or smaller than 0.05 nm.
The strain xcex94a of the (single quantum well) active layer and the strain xcex94w of the tensile strain optical waveguide layers can be expressed by
xcex94a=(caxe2x88x92cs)/cs, and 
xcex94w=(cwxe2x88x92cs)/cs, 
where ca, cw, and cs are lattice constants of the active layer, the tensile strain optical waveguide layers, and the substrate, respectively.
In this case, according to the first aspect of the present invention, the active layer and the tensile strain optical waveguide layers satisfy the inequality,
|xcex94axc2x7da+xcex94wxc2x7dw|xe2x89xa60.05 nm, 
where da and dw are respectively the total thickness of the active layer and the total thickness of the tensile strain optical waveguide layers.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (viii).
(i) The substrate may be made of GaAs, the active layer may be made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, and the optical waveguide layers may be made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, where 0.4xe2x89xa7x1 greater than 0.49y1, 0xe2x89xa6y1xe2x89xa60.1, 0 less than x3 less than 0.49y3, and 0 less than y3xe2x89xa60.5.
(ii) The semiconductor laser device according to the first aspect of the present invention may further comprise a cladding layer formed between the substrate and the active region, wherein the substrate may be made of GaAs, the cladding layer may be made of one of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 and Alz1Ga1xe2x88x92z1As, where x4=(0.49xc2x10.01)y4, 0.9xe2x89xa6y4xe2x89xa61, and 0.2xe2x89xa6z1xe2x89xa60.7.
(iii) The semiconductor laser device according to the first aspect of the present invention may further comprise a current confinement layer which is formed above the active region, and includes a groove allowing current injection into the active layer so as to realize an index-guided structure.
(iv) In the semiconductor laser device having the additional feature (iii), the groove may have a width of 1 to 4 micrometers, and the difference in the equivalent refractive index between the portion of the active layer which is located under the groove and the other portions of the active layer which are not located under the groove may be 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923.
In the internal stripe structure, when the equivalent refractive index at the oscillation wavelength in the portions of the active layer which are not located under the groove is denoted by na, and the equivalent refractive index at the oscillation wavelength in the portion of the active layer which is located under the groove is denoted by nb, the difference xcex94n in an equivalent refractive index between the portions of the active layer which are not located under the groove and the portion of the active layer which is located under the groove is expressed by xcex94n=nbxe2x88x92na.
(v) In the semiconductor laser device having the additional feature (iii), the groove may have a width greater than 4 micrometers, and the difference in the equivalent refractive index between the portion of the active layer which is located under the groove and the other portions of the active layer which are not located under the groove may be 2xc3x9710xe2x88x923 or more.
(vi) Predetermined regions of the semiconductor laser device except for a predetermined stripe region of the semiconductor laser device may be removed so that a ridge-shaped current path and an index-guided structure are realized.
(vii) In the semiconductor laser device having the additional feature (vi), the predetermined stripe region may have a width of 1 to 4 micrometers, and the difference in the equivalent refractive index between the portion of the active layer which is located under the predetermined stripe region and the other portions of the active layer which are not located under the predetermined stripe region may be 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923.
In the ridge stripe structure, when the equivalent refractive index at the oscillation wavelength in the portions of the active layer which are not located under the ridge (predetermined) stripe region is denoted by nA, and the equivalent refractive index at the oscillation wavelength in the portion of the active layer which is located under the ridge (predetermined) stripe region is denoted by nB, the difference xcex94N in an equivalent refractive index between the portions of the active layer which are not located under the ridge (predetermined) stripe region and the portion of the active layer which is located under the ridge (predetermined) stripe region is expressed by xcex94N=nBxe2x88x92nA.
(viii) In the semiconductor laser device having the additional feature (vi), the predetermined stripe region may have a width greater than 4 micrometers, and the difference in the equivalent refractive index between the portion of the active layer which is located under the predetermined stripe region and the other portions of the active layer which are not located under the predetermined stripe region may be 2xc3x9710xe2x88x923 or more.
(2) According to the second aspect of the present invention, there is provided a semiconductor laser device comprising a substrate and an active region being formed above the substrate. The active region includes an active layer and optical waveguide layers. Further, the active layer includes a plurality of quantum well sublayers and at least one barrier sublayer formed between the plurality of quantum well sublayers, and the optical waveguide layers have a predetermined amount of tensile strain and a first predetermined total thickness, and are formed so that the active layer is sandwiched between the optical waveguide layers. The plurality of quantum well sublayers have a predetermined amount of compressive strain and a second predetermined total thickness, and the at least one barrier sublayer has the predetermined amount of tensile strain and a third predetermined total thickness. In addition, the absolute value of the sum of the product of the predetermined amount of compressive strain and the second predetermined total thickness and the product of the predetermined amount of tensile strain and the sum of the first predetermined total thickness and the third predetermined total thickness is equal to or smaller than 0.05 nm.
Preferably, the semiconductor laser device according to the second aspect of the present invention may also have one or any possible combination of the aforementioned additional features (ii) to (viii) and the following additional feature (ix).
(ix) The substrate may be made of GaAs, the plurality of quantum well sublayers may be made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, and the optical waveguide layers and the at least one barrier sublayer may be made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, where 0.4xe2x89xa7x1 greater than 0.49y1, 0xe2x89xa6y1xe2x89xa60.1, 0 less than x3 less than 0.49y3, and 0 less than y3xe2x89xa60.5.
That is, according to the second aspect of the present invention, the (multiple quantum well) active layer and the optical waveguide layers satisfy the inequality,
|xcex94axe2x80x2xc2x7daxe2x80x2+xcex94wxc2x7(dw+db)|xe2x89xa60.05 nm 
where xcex94axe2x80x2 and daxe2x80x2 respectively denote the amount of compressive strain and the total thickness of the plurality of quantum well sublayer in the (multiple quantum well) active layer, xcex94w denotes the amount of tensile strain of the optical waveguide layers and the at least barrier sublayer in the (multiple quantum well) active layer, dw denotes the total thickness of the optical waveguide layers, and db denotes the total thickness of the at least barrier sublayer in the (multiple quantum well) active layer.
By the way, when c is a lattice constant of a layer formed above the substrate, and the absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.001, the layer is lattice-matched with the substrate.
(3) The present invention has the following advantages.
(a) In the semiconductor laser device according to the first aspect of the present invention, the compressive strain active region is formed above the substrate, and the compressive strain active region includes an active layer sandwiched between tensile strain optical waveguide layers. In addition, the absolute value of the sum of the product of the amount of compressive strain and the thickness of the compressive strain quantum well active layer and the product of the amount of tensile strain and the total thickness of the optical waveguide layers is equal to or smaller than 0.05 nm. Therefore, the compressive strain of the compressive strain quantum well active layer is compensated for by the tensile strain of the optical waveguide layers, and the interlayer stress produced between the active region and adjacent layers becomes small. Thus, crystal defects in vicinities of the active region can be reduced. Resultantly, the semiconductor laser device according to the first aspect of the present invention is highly reliable even in operation with high output power.
(b) In the semiconductor laser device according to the second aspect of the present invention, the active region is formed above the substrate, and the active region includes optical waveguide layers and the active layer sandwiched between the optical waveguide layers. In addition, the active layer includes a plurality of quantum well sublayers and at least one barrier sublayer formed between the plurality of quantum well sublayers. Further, the absolute value of the sum of the product of the amount of compressive strain and the total thickness of the one or more quantum well sublayers, the product of the amount of tensile strain and the total thickness of the optical waveguide layers, and the product of the amount of tensile strain and the total thickness of the at least one barrier sublayer is equal to or smaller than 0.05 nm.
Therefore, the compressive strain of the one or more quantum well sublayers are compensated for by the tensile strain of the at least one barrier sublayer, and the average strain of the active layer becomes small. Thus, the interlayer stress produced between the active layer and adjacent optical waveguide layers becomes small. Resultantly, crystal defects in the active region can be reduced.
Further, the average compressive strain of the active layer is also compensated for by the tensile strain of the optical waveguide layers. Therefore, the interlayer stress produced between the active region and adjacent layers becomes small. Thus, crystal defects in vicinities of the active region can be reduced.
Consequently, the semiconductor laser device according to the second aspect of the present invention is highly reliable even in operation with high output power.
(c) When the difference in the equivalent refractive index between the portion of the active layer which is located under the groove and the other portions of the active layer which are not located under the groove is in the range of 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923 in the semiconductor laser devices according to the present invention having an index-guided structure with a stripe width of 1 to 4 micrometers, the semiconductor laser device can emit laser light in a fundamental transverse mode even in operation with high output power.
(d) When the difference in the equivalent refractive index between the portion of the active layer which is located under the groove and the other portions of the active layer which are not located under the groove is 2xc3x9710xe2x88x923 or more in the semiconductor laser devices according to the present invention having an index-guided structure with a stripe width greater than 4 micrometers, the semiconductor laser device can emit highly reliable laser light with low noise even in multiple modes.